An alliance official termed the Belarusian regime's ploy "a hybrid tactic", meaning a combined military-political operation, and said: "NATO stands ready to further assist our allies, and maintain safety and security in the region".

Belarus, allied with Russia, is under EU sanctions after diverting a Ryanair flight between EU capitals in May to arrest a dissident Belarusian journalist and his girlfriend on board, and brutally cracking down on the opposition.

Authorities in EU member Poland said on Monday that Belarus was sending hundreds of migrants to the border, where it has built high rows of barbed wire and deployed troops to hinder illegal crossings.

The EU has accused Belarusian strongman Alexander Lukashenko of facilitating the wave of migrants and refugees, mainly from the Middle East, in retaliation for the sanctions.

A Polish emergency law has barred media from reporting along the border, where at least 10 migrants have died, apparently blocked from entering further into Poland and stopped from going back into Belarus.

"We are concerned by the recent escalation at the border between Poland and Belarus," the NATO official said in a statement, remaining anonymous in line with the alliance's practice.

"We call on Belarus to abide by international law. We have seen a surge of migrants trying to enter Allied territory via Belarus.

"NATO continues to closely monitor the situation, which is putting pressure on our Allies Lithuania, Latvia and Poland," the official said.

- 'Inhumanly misusing people' -

The European Commission cited Frontex, the EU's border security agency based in Poland but not involved in policing its border with Belarus, saying "several hundred" people were at the frontier.

"This is a continuation of the desperate attempt by Lukashenko regime to use people as pawns to destabilise the European Union," a spokesman, Adalbert Jahnz, told journalists.

Another spokesman, Peter Stano, suggested EU countries were discussing further sanctions against Belarus.

"We are facing a hybrid threat and we will not stand without any action," he said, adding that the issue would be taken up by EU foreign ministers meeting in a week's time.

Sanctions deliberations were confidential, but the EU was treating as "a matter of urgency" the "unacceptable" situation Belarus was foisting upon the EU's borders, he said.

Stano added that the EU was speaking to countries where the migrants were departing from to warn them "how the Lukashenko regime is brutally, inhumanly misusing people -- their own citizens -- and bringing them into very difficult situations".

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